This is a method I literally just came up with (3/28/07) and so I added a link to my Tidbits page as a result! Hover below this text. Be careful, though. I've noticed some different browsers doing things differently. After the image appears, IE requires that you click off the image once before alternating between hovering on and off the picture makes it consistently appear and disappear (swap).
IE-based browsers, I can technically go from non-graphic to graphic in the swapping.
Gecko-based browsers (Firefox, etc.) aren't so forgiving. The main advantage of this
disadvantages are the following: less control, less ability to do more dynamic things
combined with swapping, and image dimensions must be set for it to work properly (while I
the discovery of the hidden image easier when hovering.
To figure it out, view the source. I've put in plenty of comments.